These low-thrust motors make it possible to limit the mass of fuel necessary to perform the satellite orbit transfer operation. However, these motors being low power, they exhibit the drawback of lengthening the time required for placement on station or for orbital transfer by one to two orders of magnitude with respect to the use of high-thrust chemical motors. The nominal duration of the orbital transfer at low thrust may indeed vary from a few weeks to a few months.
On account of this low power and of the lengthening of the transfer time or the time required for placement on station, the control procedures which determine the motor thrust law (direction and amplitude as a function of time) and which are used for high-thrust chemical motors, are not applicable for low-thrust motors.
A control procedure for electric motors is described in the publication “Boeing Low-Thrust Geosynchronous Transfer Mission Experience”, for orbital transfer from an elliptical injection orbit delivered by a launch vehicle, to a geostationary target orbit. It consists in the course of a first phase in applying a continuous thrust along the instantaneous velocity vector of the satellite until the latter attains an elliptical orbit of the same period as that of the target orbit. A second phase is devoted to the transformation of this elliptical orbit into a circular orbit by using a law of thrust orientation perpendicular to the apogee-perigee line in the plane of the orbit. This procedure exhibits a few drawbacks:
it is sub-optimal in so far as the transfer time is too long and the electrical fuel consumption (Xenon, Argon, etc.) too great;
it does not make it possible to attain the circular target orbit such as the operational orbit with sufficient precision;
it is limited to a transfer of GTO-GEO type, that is to say to a transfer from an elliptical orbit to a circular orbit of period 24 h. Furthermore it is also possible to envisage a transfer from a non-elliptical orbit to a non-circular orbit or more generally a transfer whatever the satellite starting orbit and arrival orbit.
These drawbacks are overcome by the method of placing on station described in patent application FR 2998875. It can be carried out onboard the satellite (in particular having requirements in terms of memory and calculation resources which are compatible with the performance of a satellite). This method makes it possible to determine the optimal control law whatever the starting and arrival orbit of the satellite, while minimizing journey time or fuel consumption when placing the satellite on station or during its orbital transfer. The resources, in terms of amount of memory available and of calculation power which are necessary for the operation of the method, are low with respect to the computing resources of current satellites. The control procedure is robust to mission interruptions, such as the interruption of steering for maintenance, faults, etc. The control procedure is capable of automatically correcting in closed-loop the optimal control law as a function of the deviation from the nominal trajectory, with simple calculations and without re-programming from the ground. Finally, this solution allows the achieving of autonomous orbital transfer and is suited to the use of electric motors.
But this method which is based notably on the knowledge onboard the satellite and in real time, of the position of the satellite, requires that the latter be equipped with a receiver of GNSS type. Such a receiver is difficult to design since the acquisition of the information is carried out on the sidelobes of the antenna of the receiver and therefore with a low SNR. And such a receiver is not suited to orbits or portions of orbit whose altitude is greater than that of the constellation of GNSS satellites which is about 20 000 km. Moreover, this method which is particularly well suited to the placement on station of a satellite in self-rotation about the thrust vector, poses an implementational problem when the satellite is not in this configuration.